Prank Idea! Answered
A while ago I read an 'ible about how to turn on your schools sprinkler system as a last day prank.
What about discreetly installing a remote activator thing in the valve boxes, to turn on all of the sprinklers, inpependently of whatever is there already - rc system, 12v battery, relay, diodes. Over the year you could do this to every valve, and trigger them right at the end.
Unfortunately, living in the UK, no schools have sprinkler systems - somewhere where they do, someone try this!